This invention relates to a connector adapter for receiving a connector plug terminating a polarization maintaining (PM) optical fibers.
In optical fiber communications, connectors for joining fiber segments at their ends, or for connecting optical fiber cables to active or passive devices, are an essential component of virtually any optical fiber system. The connector or connectors, in joining fiber ends, for example, has, as its primary finction, the maintenance of the ends in a butting relationship such that the core of one of the fibers is axially aligned with the core of the other fiber so as to maximize light transmissions from one fiber to the other, or, put another way, to reduce insertion loss. Another goal is to minimize back reflections. Alignment of these small diameter fibers is extremely difficult to achieve, which is understandable when it is recognized that the mode field diameter MFR of, for example, a singlemode fiber is approximately nine (9) microns (0.009 mm). The MFR is slightly larger than the core diameter. Good alignment (low insertion loss) of the fiber ends is a function of the transverse offset, angular alignment, the width of the gap (if any) between the fiber ends, and the surface condition of the fiber ends, all of which, in turn, are inherent in the particular connector design. The connector must also provide stability and junction protection and thus it must minimize thermal and mechanical movement effects.
In the present day state of the art, there are numerous, different, connector designs in use for achieving low insertion loss and stability. In most of these designs, a pair of ferrules (one in each connector), each containing an optical fiber end, are butted together end to end and light travels across the junction. Zero insertion loss requires that the fibers in the ferrules be exactly aligned, a condition that, given the necessity of manufacturing tolerances and cost considerations, is virtually impossible to achieve, except by fortuitous accident. As a consequence, most connectors are designed to achieve a useful, preferably predictable, degree of alignment, some misalignment being acceptable.
However, in connecting or terminating polarization maintaining (PM) fibers, such is not the case. Many optical fiber components, such as, for example, interferometers and sensors, lasers, and electro-optic modulators, are extremely sensitive to and dependent upon, for proper operation, the polarization of the light. Even very slight alterations or changes in the light polarization orientation can result in wide swings in the accuracy of response of such devices. PM fiber has polarization-dependent refractive indices, and the speed of light in an optical fiber is inversely proportional to the magnitude of the refractive index. A birefringent optical fiber is one having two polarizations having different velocities of propagation, thus giving rise to a xe2x80x9cfastxe2x80x9d wave and a xe2x80x9cslowxe2x80x9d wave. In a PM fiber, the polarization of a linearly polarized light wave input to the fiber, with the direction of polarization parallel to that of the one of the two principal polarizations, will remain or be maintained in that polarization as it propagates along the fiber, hence the term xe2x80x9cpolarization maintaining.xe2x80x9d If the polarization of the light wave is to be maintained at a splice or other connection, the principal axes of birefringence of the two joined fibers must be aligned in parallel, otherwise there will be polarization cross-coupling, i.e., crosstalk, which is highly undesirable. Thus, where two PM fibers, for example, are to be connected together, they should be terminated carefully to reduce the crosstalk during the connectorization process. Also, the connectors must be capable of aligning then maintaining the fiber orientation to the connector key position. Connectors with tolerances adequate for connecting non-PM fibers usually are inadequate for maintaining polarization alignment at the connector junction.
Typical PM connector requirements are an insertion loss of less than 0.3 dB, and the prior art PM connector arrangements comprise numerous, different connector configurations aimed at meeting these requirements for different connectors, such as an SC type connector as shown in U.S. Pat. No. 5,216,733 of Ryo Nagase et al. The connector of that patent comprises a ferrule body and a ring shaped flange having a keyway mounted on the periphery of the ferrule body. Alignment is achieved by rotating the ferrule body with respect to the flange keyway. The combination of ferrule and flange comprises a plug which is inserted into a push-pull SC connector having a key therein for mating with the flange keyway and springs bias the flange in the longitudinal direction to maintain the alignment.
In U.S. Pat. No. 4,784,458 of Horowitz, a splice joint for PM fibers is shown wherein aligned fibers are joined with UV curing epoxy, and the joint is overlaid with epoxy cement for rigidity. Such a joint is permanent, and does not function as a connect-disconnect optical fiber connector.
U.S. Pat. No. 5,561,726 of Yao discloses an apparatus for controlling the polarization state of the light within a fiber by squeezing a portion of the fiber to produce a birefringent fiber, and the squeezer is then rotated to change the polarization of the light within the fiber. The device is not a connector, but is intended for use with polarization sensitive devices such as interferometers and electro-optic modulators, however, it may also be used with connectors for connecting two PM fibers.
It is common practice in the prior art for creating PM fibers to include a pair of rods in the fiber cladding which extend parallel to the core as shown in U.S. Pat. No. 4,515,436 of Howard et al. Such rods, which are preferably of glass, are, in manufacture of the fiber, included in the fiber preform from which the fiber is drawn. As the fiber is drawn, the rods are accordingly diminished in diameter and are located within the cladding, preferably on either side of the core. The rods have different thermal expansion characteristics than the surrounding glass, and the stress they exert on the core causes the index of refraction to change along that axis. The axes then have different indices of refraction value and thus propagate light at different speeds. Variations on the two rod arrangement are also known, such as the elliptical stress member disclosed in U.S. Pat. No. 5,488,683 of Michal et al. Also, squeezing the fiber to create birefringence, as shown in the aforementioned Yao patent is feasible. The two rod PM fiber, so called xe2x80x9cPandaxe2x80x9d type PM fiber, however, has proven quite satisfactory in use, and it is toward the connectorization of such a fiber that the present invention is directed, although other types of PM fibers may be used with the present invention.
In the copending U.S. patent application Ser. Nos. 10/151,613 and 10/151,450 are shown, respectively, a PM connector plug and an adapter therefor the principles of which are applicable to any of a large number of optical fiber connectors, but are embodied in a modified LC connector in those applications and a method and apparatus for tuning the connector. For optimum performance, i.e., maximum transmission of a polarized beam, it is highly desirable to provide accurate rotational positioning of better than xc2x11xc2x0 or even as accurate a  less than xc2xcxc2x0 between connectors equipped with polarization maintaining fibers.
The present invention is an adapter fore receiving the connector plug of application Ser. No. 10/151,613. When a PM jumper cable, for example, is terminated by connectors, it is most desirable that the cable/connector combination be tuned to align the fiber slow axis with the connector key which serves as a reference point. In accordance with the present invention, there is provided an adapter forming, with the connector plug, a PM connector.
The adapter of the present invention is, for purposes of illustration, basically an LC type adapter modified to receive the PM connector plug of Lampert et al. application Ser. No. 10/151,613 and functions to maintain the tuning position of the tuned plug, allowing both insertion and removal of the plug without altering the tuning thereof. To this end, the adapter, which preferably is made of suitable plastic material has an opening with first and second side walls for receiving the plug. At least one of the side walls has formed therein a cantilevered spring member which is biased inwardly toward the interior of the adapter. When the plug is inserted, the spring member is cammed outwardly to admit the plug which, after insertion, is laterally maintained in a proper position by the spring force.
The opening has a longitudinal slot or channel therein for receiving the latching arm of the plug, and the side walls of the slot are angled to match the angled configuration of the latching arm of the plug. Thus a cross-section of the slot has a truncated wedge shape. The slot, so angled, prevents lateral shifting or float of the latching arm and further serves to center the plug within the adapter.
The exterior of the adapter housing has a flange thereon for bearing against a panel member, and a metallic member straddles the adapter housing. The metallic member has first and second cantilever leaf spring locking members projecting outwardly for bearing against the opposite side of the panel from the flange.
The adapter of the invention in usable with conventional LC connector plugs, with the latching arm receiving slot serving to reduce transverse float inasmuch with sidewalls of the slot will bear against the straight sided latching arm at some point thereon.